The invention concerns generally the technology required for changing the data rates during an active connection between a base station and a mobile station in a radio communications system. Especially the invention concerns a situation where the code rate and/or the modulation method is changed.
EDGE or xe2x80x9cEnhanced Datarates for GSM Evolutionxe2x80x9d, where GSM stands for xe2x80x9cGlobal System for Mobile telecommunicationsxe2x80x9d, is an international project where means are developed for providing users with higher data rates in a telecommunications system based on known GSM technology. The raw bit rate at the GSM air interface between a base station and a mobile station is 22.8 kbit/s. Channel coding and the overheads associated thereto lower the usable data rate so that data rates from 3.6 to 14.5 kbit/s are available to connected data applications. Even these rates carry a certain overhead on top of the available user data rate. At the time of filing this patent application EDGE aims at a raw air-interface bit rate of 69.2 kbit/s. The enhancement in data rates is largely due to the 8PSK (8-level Phase Shift Keying) modulation method which is employed in EDGE, in contrast to the GMSK (Gaussian Minimum Shift Keying) modulation used in GSM.
It is expected that EDGE connections will not be supported throughout a cellular network. EDGE connections will most probably be available only in central areas like office buildings and densely trafficated business centres. To take advantage of the full capability of the system a mobile station must therefore support both the advanced EDGE data rates and the conventional GSM data rates. A mechanism must thus be developed for changing between the two. In EDGE, the concept of link adaptation has been defined. It comprises all means of changing the radio interface data rate during an active connection by changing the modulation method or also by changing the amount of employed channel coding. The latter may be varied to compensate for changes in the quality of the radio environment. There exists a risk that link adaptation will cause perceivable artefacts, like clicks or silent periods in an audio communication or blank or distorted pixels or fields in a video communication. It is naturally desirable that a user will not be able to perceive an executed link adaptation command by just observing the results (sound, picture etc.) of the communication. A seamless change in the radio interface data rate is defined to be a link adaptation operation which goes totally unnoticed by the user.
As a background for the invention, the transmission chain used in the conventional GSM system will be briefly discussed with reference to FIG. 1. The transmission of full-rate speech is used here as an example of a typical service requiring a circuit-switched connection. Speech recorded by a microphone 101 will first be encoded in a speech encoder 102 which converts an analogue speech signal into digital form and performs a group of encoding operations. The output signal of the speech encoder has a rate of 13 kbit/s and consists of blocks of 260 bits, the blocks following each other at an interval of 20ms. The channel encoder 103 introduces redundancy into this data flow, increasing its rate by adding into it information calculated from the contents of the blocks. The reason for channel coding is to allow the detection or even the correction of signal errors introduced later during transmission. The output of the channel encoder 103 consists of code words of 456 bits each. Exactly one code word is produced from each block of input information for the channel encoder.
The code words that come from the channel encoder 103 are input to the interleaver/burst formatter 104 for mixing up the bits of several code words in a predetermined fashion and organising them into bursts. The aim of interleaving is to decorrelate errors that will potentially occur in the transmission so that the resulting erraneous bits will be distributed into essentially randomised positions in several code words instead of corrupting a sequence of successive bits in a single code word. Most interleaving methods that are currently used are diagonal, meaning that bits from consecutive code words are cross-distributed so that certain bits of the later codeword come earlier in the interleaved data stream than certain other bits of the former codeword. In GSM, the bits from a certain code word are spread over a period of 22 bursts, and a single burst may contain bits from as many as five different code words. The interleaving details of GSM depend on the nature of the information to be interleaved (speech, data, access request etc.)
The burst formation part of the interleaver/burst formatter 104 takes a block of 116 interleaved bits and adds three zero bits (called the tail bits) at the beginning and end of the block as well as a so-called training sequence of 26 bits exactly in the middle of the block. At the output of the interleaver/burst formatter 104 the flow of information consists of formatted blocks known as bursts. For the description to be consistent throughout this patent application, the bits of a GSM burst will be called symbols in the following. Additionally the burst will be denominated as a digital burst while it is still in digital form. Each digital burst thus comprises 114 symbols of coded data and 2 so-called stealing flag symbols that indicate whether the coded data symbols include user data or signalling data belonging to a certain Fast Associated Control Channel (FACCH). Additionally each digital burst comprises the above-mentioned 3+3 tail symbols and the training sequence of 26 symbols.
The ciphering block 105 performs a logical exclusive-or operation between the coded data symbols of a digital burst and a certain pseudo-random bit sequence in order to impede the unauthorised reception of the transmitted data. The tail symbols, the stealing flag symbols and the training sequence are not ciphered. After ciphering the digital bursts are input into a modulator/upconverter 106 that transforms each digital burst into a sequence of a radio-frequency analogue oscillating signal, which is amplified in an amplifier 107 and conducted into an antenna 108 for transmission. Because of its close connection with the digital burst, the analogue signal sequence is also known as a burst; for clarity it can be further specified as a transmission burst. Several filtering operations take place inside the modulator/upconverter 106 and between it and the antenna 108; for graphical clarity the respective filter blocks are omitted from FIG. 1. In GSM a Time Division Multiple Access (TDMA) scheme is applied, in which each speech channel may use a single time slot in a cyclically repeated frame of eight consecutive time slots. The transmitter transmits one transmission burst in each time slot during the active connection.
A receiver chain for receiving, demodulating and decoding the data transmitted by the transmission chain of FIG. 1 would consist of a receiving antenna for receiving the radio signal, some filters and amplifiers for filtering and amplifying the received signal, a downconverter/demodulator or an equalizer for converting the transmission burst into digital form on baseband frequency, a deciphering block for converting the ciphered bits into plain data, a burst deconstructing/de-interleaving block for exctracting the data bits and removing the interleaving, a channel decoder for removing the channel coding, and a speech decoder/D/A converter for converting the decoded digital signal into an analogue signal from which the original speech may be reproduced by a loudspeaker. The operation of the blocks in the receiver chain is approximately the inverse of that of the respective blocks in the transmitter chain.
Minor changes are required in the above-explained functions of the transmission and reception chain blocks for other transmission modes than full-rate speech. These changes are known to the person skilled in the art from the GSM specifications published by ETSI (European Telecommunications Standards Institute) and e.g. from the book Michel Mouly, Marie-Bernadette Pautet: xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, published by the authors, ISBN 2-9507190-0-7, Palaiseau 1992.
The transmission chain of FIG. 1 is basically applicable also for EDGE transmissions, although the use of higher data rates would necessitate changes in the function of the blocks. Data requiring a higher data rate would most probably originate from a different source than a microphone and a speech encoder, for example a camera and a video encoder. The channel encoder block would operate according to the EDGE channel encoding scheme and, together with the interleaver/burst formatter, ciphering block and the modulator part of the modulator/upconverter, it would have to operate much faster than in basic GSM. The channel encoder block would also be capable of changing the amount of applied channel encoding according to link adaptation commands.
The most radical difference would result from the different modulation method. In the 8PSK modulation scheme of EDGE, three consecutive bits in the formatted digital burst are mapped onto one transmission symbol. For this reason already a symbol in the digital burst is said to consist of a group of three consecutive bits instead of one bit as in GSM. FIG. 2 illustrates the mapping principle. A transmission symbol is a sequence of an oscillating analoque signal with constant amplitude and frequency and with a phase of i(xcfx80/4) radians, where i takes an integral value from 0 to 7 depending on the values of the three bits to be mapped. Dots in the phase diagram of FIG. 2 mark the allowed end positions in the complex plane for the vector representation of the analogue signal xcfx86. The axes are the real part Re(xcfx86) and the imaginary part Im(xcfx86) of the signal. During the transmission of a burst, the transmitter will produce transmission symbols with the instantaneous rate of 270 ksymbols/s, which is the same as in GSM; the difference in performance results from the fact that an 8PSK symbol carries the information equivalent to three bits, whereas in GMSK each symbol only corresponds to one bit.
The problem concerned by the present invention arises when the applied modulation method is abruptly changed from GMSK to 8PSK or vice versa, and/or when the amount of applied channel coding is abruptly changed. If the transmitter waits until the end of the current interleaving period before commissioning the change, there will be a time interval with practically no transmitted data and consequently a blackout in the circuit-switched communications service. The end of the current interleaving period is defined as the moment when all such interleaved bits have been transmitted that were encoded using the xe2x80x9coldxe2x80x9d channel encoding method and/or modulated using the xe2x80x9coldxe2x80x9d modulation method. The nature of interleaving states that towards the end of such interleaving period the transmitted bursts will be mostly empty. The same applies to the beginning of the first interleaving period with the xe2x80x9cnewxe2x80x9d channel encoding and/or modulation.
It is therefore an object of the present invention to present a method for changing between modulation and/or channel encoding methods in a way that eliminates the silent period or seam caused by interleaving in prior art solutions. It is also an object of the present invention to present suitable transmitter and receiver apparatuses for performing the method of the invention.
The objects of the invention are fulfilled by applying the same interleaving order regardless of channel encoding and modulation, and by always using the higher-order modulation during a certain transition period between two differently ranked modulation methods.
The method according to the invention is characterised in that it comprises the steps of
a) for the construction of a certain symbol, finding the code words the bits of which have influence on the content of the symbol under construction and
b) constructing the symbol under construction according to a predetermined formula,
wherein said predetermined formula is chosen according to the length of the code words the bits of which have influence on the content of the symbol under construction.
The transmitter according to the invention is characterised in that it is arranged to operate according to said method.
The receiver according to the invention is characterised in that it is arranged to operate according to said method.
According to the invention the interleaving operation is defined so that each symbol in a digital burst to be formatted has an unambiguously defined origin among the bits of the code words irrespective of the lengths of the code words. In a link adaptation operation where the length of the code words changes, the interleaver/burst formatter of the transmitter produces each symbol according to a specific rule that depends on the length of the code word where the origins of the symbol are. As a consequence, the bits from consecutive code words may be interleaved and mapped into the symbols of the digital bursts regardless of the amount of applied channel coding.
A change in modulation order, accompanied by the regularly continuing interleaving explained above, will result in a transition period during which a transmission burst will contain some previously diagonally interleaved symbols that were, at the time of their generation, meant to be transmitted with the old modulation method. The rest of the symbols in these transmission bursts have been generated for transmission with the new modulation method. According to the invention the higher-order modulation method will be used for transmitting each whole burst containing symbols that were generated for transmission with the higher-order modulation method. Some symbols that were generated for transmission with the lower-order modulation method will thereby be transmitted with the higher-order modulation method, but this does not cause significant disadvantages.
In EDGE, a communications service may have at its disposal more than one time slot in a frame. A change in modulation order will necessitate a change in the number of used time slots, if it is not accompanied with a simultaneous change in the amount of channel coding that would compensate for the growing/decreasing need for transmission time at the radio interface. For example a 28.8 kbps service may first have one 28.8 kbps slot with 8PSK modulation, and after the change two 14.4 kbps slots with GMSK modulation, or vice versa. During a transition period the service may have two time slots in a frame at its disposal, and two consecutive transmission bursts to be transmitted, the transmission bursts being a xe2x80x9c8PSK-burstxe2x80x9d and a xe2x80x9cGMSK-burstxe2x80x9d. Of these, the former is a burst containing some symbols generated for transmission with 8PSK and interleaved among a number of symbols generated for transmission with GMSK. The latter is a burst containing only symbols generated for transmission with GMSK. The most advantageous solution, from the point of view of radio network performance, would be to transmit the xe2x80x9c8PSK-burstxe2x80x9d with 8PSK in the first available time slot of the frame and the xe2x80x9cGMSK-burstxe2x80x9d with GMSK in the second available time slot of the frame. However, this would require the transmitter to change modulation between two consecutive time slots in a frame, which set rather hard requirements for the operation of the transmitter. The operational requirements may be considerably eased by allowing for the transmission of all transmission bursts in the available time slots in a frame with the higher-order modulation method whenever there is at least one xe2x80x9c8PSK-burstxe2x80x9d to be transmitted in that frame.
The invention has been described above exclusively with references to EDGE, GSM, 8PSK and GMSK. However, it is clear to the person skilled in the art that the invention is not in any way limited to these particular definitions but it can be applied in all radio communications systems where a change between modulation methods of different orders and/or a change in the amount of applied channel coding is allowed during an active connection, and where diagonal interleaving is employed.
The invention provides for seamless transition between modulation methods and between various amounts of applied channel coding, which was the object set for the invention.